The Arbiter was a "Heretic" Elite blamed for the destruction of the first Halo discovred by the UNSC. An Arbiter is someone who is decides, and enforces and issue. The past Arbiter's of the Covenant have dealt with such situations such as those in campaign mode of Halo 2. (By the way, incase none of you pin-heads noticed, the Arbiter's voice is the same of 'Golaith' from Gargoyales.)

The Arbiter was ordered to deal with the true heretics of the Covenant